Employee Benefit Plans (Quarterly Info) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Pension Benefits [Member]
Components of net periodic benefit cost (credit):
Service cost	$ 11	$ 14
Interest cost	12	11
Expected return on plan assets	(15)	(16)
Amortization of net actuarial loss	4	6
Net periodic benefit cost (credit)	12	15
Estimated future employer contributions in current fiscal year	63
Other Postretirement Benefits [Member]
Components of net periodic benefit cost (credit):
Interest cost	2	2
Expected return on plan assets	(2)	(3)
Amortization of prior service cost (credit)	0	(1)
Reclassification to regulatory liability	0	1
Net periodic benefit cost (credit)	0	(1)
Amortization of prior service cost (credit) from regulatory assets (liabilities)		$ (1)
Employer contributions	1
Estimated future employer contributions in current fiscal year	$ 4